Foamed mechanical planarization pads made with supercritical fluid

ABSTRACT

Foamed thermoplastic polymeric mechanical planarization polishing pads (“MP pads”) made with supercritical fluids are presented. A supercritical fluid foaming agent is dissolved in a thermoplastic polymer. A rapid change in the solubility and volume of the supercritical fluid foaming agent in the thermoplastic polymer results in foaming of the thermoplastic polymer. Foamed thermoplastic polymeric MP pads are advantageously both significantly and uniformly porous.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of commonly-assigned U.S. patent application Ser.No. 10/180,408, filed Jun. 24, 2002, now U.S. Pat. No. 7,166,247, whichis hereby incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

This invention relates to uniformly-foamed thermoplastic polymericmechanical planarization polishing pads (“MP pads”). More particularly,this invention relates to uniformly-foamed thermoplastic polymeric MPpads made with supercritical fluids.

Multiple layers of conducting, semiconducting, dielectric, andinsulating materials are deposited on a substrate during integratedcircuit device fabrication. Often, imperfect substrate fabrication andimperfect integrated circuit layer deposition result in formation ofundesirable topography (e.g., recesses, protrusions, scratches, etc.) onthe substrate and on one or more of the deposited layers. Becauseundesirable topography can compromise the integrity of an integratedcircuit device (e.g., a topographical recess in a dielectric layer canimpose step coverage problems with the deposition of another integratedcircuit layer, undesirable topology can cause depth of focus issuesduring photolithography, etc.), the substrate and each deposited layerof an integrated circuit device are preferably planarized (i.e., madelevel) before additional layers of integrated circuit material aredeposited.

Known mechanical planarization (“MP”) processes are used to removeundesirable topology from layers of integrated circuit material.Generally, an MP pad rotating about a line preferably perpendicular tothe surface of an integrated circuit wafer is brought into contact withthat surface during an MP process. The rotating MP pad mechanicallypolishes (i.e., removes undesirable topography from) the surfacematerial of the integrated circuit wafer. Concurrently, a fluid-basedchemical (i.e., a chemical polishing “slurry”) that reacts with theintegrated circuit material (i.e., for a chemical-mechanicalplanarization (“CMP”) process) or an inert liquid applied to the MP padfacilitates the removal of undesirable topography. For example, an inertliquid applied to the interface between an MP pad and an integratedcircuit wafer can facilitate the removal of mechanically-groundintegrated circuit material.

The porosity of an MP pad is often controlled to positively affect thematerial removal rate of an MP process. In particular, the porositylevel of an MP pad directly influences and can increase the materialremoval rate, because the “pores” of an MP pad retain and distributechemical or inert polishing liquid that facilitates the planarization ofundesirable topography. However, a significantly porous MP pad may beundesirable unless the MP pad pores are both uniform in size and uniformin distribution throughout the MP pad. Because uniform MP pad poresevenly distribute polishing liquid to the surface of an integratedcircuit wafer, a uniformly porous MP pad contributes to a desirableuniform material removal rate across the surface of the integratedcircuit wafer. Thus, uniformity in the porosity level of the MP pad(i.e., uniformity in porosity level across the surface and throughoutthe bulk of a single MP pad and uniformity in porosity level from MP padto pad) is an important MP pad characteristic.

Various known fabrication methods produce porous MP pads. For example,the known method of including hollow microbeads in a liquid prepolymerimparts porosity in thermoset polymer MP pads (e.g., Rodel IC1000 MPpad). As another example, the known method of coating a porous networkof felt or woven fibers with a thermoset polymer also imparts porosityin thermoset polymer MP pads (e.g., Thomas West 711 MP pad). In anotherknown method, perforations (i.e., slurry “cups”) or through-holes arecut or molded into a polymer to provide porosity in MP pads. In stillanother known method, direct foaming of thermoset polymers using anon-supercritical fluid foaming agent produces porous thermoset polymerMP pads (e.g., Universal Photonics ESM-U MP pad).

Thermoset polymer MP pads may be, however, problematic. In particular,because thermoset polymers are generally formed in thick “cakes” thatare characteristically non-uniform over the surface and throughout thebulk of the cake (which is caused by a non-uniform temperature of thecake during curing of the thermoset polymer), individual thermosetpolymer MP pads mechanically skived (i.e., cut) from a thermoset cakeare likely to exhibit unpredictable irregularities and non-uniformity.Further, mechanically skiving a thermoset cake can introduce surface andbulk irregularities such as, for example, fracturing and abrasions inthermoset MP pads. Thus, thermoset polymer MP pads are oftencharacterized by undesirable non-uniformity across the surface andthroughout the bulk of a single MP pad and by undesirable non-uniformityfrom MP pad to pad.

Because non-uniform MP pads can produce undesirable non-uniformity inthe surface of an integrated circuit wafer during polish of that wafer,it may not be desirable to use caked thermoset polymer MP pads in an MPprocess. In addition, because mechanically cutting thermoset cakes toproduce thermoset MP pads typically results in significant materialwaste (i.e., the unusable material cut from the edges of thermosetcakes), methods of fabricating thermoset MP pads from caked thermosetpolymers may not be cost-effective. Note that although single thermosetpolymer MP pads may be formed via reaction injection molding (“RIM”),difficulty in controlling the ratio of components of the thermosetpolymer during injection and in controlling the temperature of thethermoset polymer during thermoset polymer curing causes these pads tobe especially non-uniform.

In contrast to thermoset polymers that are generally formed in thickcakes, thermoplastic polymers are generally formed (e.g., molded orextruded) in single sheets or units at a time. Thus, thermoplasticpolymeric MP pads can be advantageously individually fabricated andgenerally do not require mechanical skiving that can cause MP paddefects and material waste.

For example, Cook et al. U.S. Pat. No. 6,325,703 describes a method offabricating porous thermoplastic polyurethane MP pads by sintering. Inparticular, dry thermoplastic polyurethane resins are placed in anindividual MP mold and “welded” together via a heating cycle (attemperatures below the melting point) to produce a porous thermoplasticpolyurethane MP pad. However, sintered thermoplastic polymeric MP padsmay be problematic. In particular, because dry thermoplastic resins areoften imperfectly mechanically ground to a predetermined size beforethey are sintered, and because slight variations in resin size canresult in undesirably non-uniform pores, sintered thermoplasticpolymeric MP pads can be undesirably non-uniformly porous. Further,uneven pressure and uneven distribution of dry thermoplastic resins inan MP pad mold can result in sintered thermoplastic polymeric MP padsthat are non-uniformly porous.

As another example, Budinger et al. U.S. Pat. No. 4,927,432 describes amethod of coalescing a solubilized thermoplastic polymer with a porousnetwork of felt or woven fiber to impart porosity in thermoplasticpolymer pads. However, because the thermoplastic pad derives itsporosity from the projecting ends of the porous network, and becausethese projecting ends are somewhat randomly distributed, thermoplasticMP pads made by coalescing thermoplastic polymer with felt or wovenfiber are often non-uniformly porous.

Other products (e.g., polystyrene packaging, high density polyethylenebottles, etc.) use known fabrication methods to produce porousthermoplastic polymeric materials that are both significantly porous anduniform in porosity. In particular, known fabrication methods usingsupercritical fluids produce foamed thermoplastic polymeric materials(which are characteristically porous) that are both significantly porousand uniform in porosity. For example, methods of fabricating foamedthermoplastic polymeric materials using supercritical fluids aredescribed in Cha et al. U.S. Pat. No. 5,158,986, Park et al. U.S. Pat.No. 5,866,053, Blizard et al. U.S. Pat. No. 6,231,942, Park et al U.S.Pat. No. 6,051,174, and Blizard et al. U.S. Pat. No. 6,169,122. In theknown methods, a rapid change in the solubility and volume of asupercritical fluid dissolved in a thermoplastic polymer results infoaming of the thermoplastic polymer. Moreover, because thermoplasticpolymeric scrap material can be reprocessed, these known methods offabricating foamed thermoplastic polymers using supercritical fluids caneliminate process waste of thermoplastic polymeric material. However,such methods are not known for fabricating MP pads.

In view of the foregoing, it would be desirable to use known methods offabricating foamed thermoplastic polymeric materials using supercriticalfluids to fabricate foamed thermoplastic polymeric MP pads.

SUMMARY OF THE INVENTION

It is an object of this invention to use known methods of fabricatingfoamed thermoplastic polymeric materials using supercritical fluids tofabricate foamed thermoplastic polymeric MP pads.

In accordance with the invention, known methods are used to make foamedthermoplastic polymeric MP pads that are both significantly porous anduniform in porosity. In these methods according to the invention, asupercritical fluid foaming agent is dissolved in a thermoplasticpolymer. A rapid decrease in solubility of the supercritical fluidfoaming agent in the thermoplastic polymer and a rapid increase involume of the supercritical fluid foaming agent in the thermoplasticpolymer result in foaming of the thermoplastic polymer. In someembodiments, one or more pressure drops cause the rapid changes insolubility and in volume of the supercritical fluid foaming agent. Inother embodiments, one or more temperature increases cause the rapidchanges in solubility and volume of the supercritical fluid foamingagent. In still other embodiments, one or more pressure drops and one ormore temperature increases cause the rapid changes in solubility andvolume of the supercritical fluid foaming agent in the thermoplasticpolymer. The rate of pressure drop and the rate of temperature increaseare directly proportional to cell density in a foamed thermoplasticpolymer. Produced thermoplastic polymeric MP polishing pads can beopen-celled or closed-celled in accordance with the invention.

In one method of fabricating foamed thermoplastic polymeric MP polishingpads in accordance with the invention, a foamed thermoplastic polymericMP polishing pad is produced by foaming a solid thermoplastic polymerimpregnated with a supercritical fluid foaming agent. In one embodimentof this method, the solid thermoplastic polymer is molded in an MP padmold before it is foamed. In other embodiments, the solid thermoplasticpolymer is formed into a sheet from which individual MP pads aremechanically skived (either before or after the solid thermoplasticpolymer is foamed).

In another method of fabricating foamed thermoplastic polymeric MPpolishing pads in accordance with the invention, a foamed thermoplasticpolymeric MP pad is produced by foaming a single-phase solution of athermoplastic polymer and supercritical fluid foaming agent. In oneembodiment of this method, the foamed thermoplastic polymer is molded inan MP pad mold. In other embodiments, the foamed thermoplastic polymeris formed into a sheet from which individual MP pads are mechanicallyskived. Percent weight by composition of the supercritical fluid foamingagent in the single-phase solution is directly proportional to celldensity in the foamed thermoplastic polymer.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the invention will beapparent upon consideration of the following detailed description, takenin conjunction with the accompanying drawings, in which like referencecharacters refer to like parts throughout, and in which:

FIG. 1 is a flowchart of an exemplary method of fabricating foamedthermoplastic polymeric MP pads using supercritical fluid according tothe invention;

FIG. 2 is a graph of cell densities for an exemplary foamedthermoplastic polymer versus percent composition by weight of anexemplary supercritical fluid foaming agent in a single-phase solutionof the corresponding thermoplastic polymer and supercritical fluidfoaming agent;

FIG. 3 is a three-dimensional graph of the solubility of a supercriticalfluid foaming agent in a thermoplastic polymer versus processtemperature and versus process pressure;

FIG. 4 is a graph of cell densities for an exemplary foamedthermoplastic polymer versus pressure drop rates of a single-phasesolution of the corresponding thermoplastic polymer and an exemplarysupercritical fluid foaming agent;

FIG. 5 is a flowchart of another exemplary method of fabricating foamedthermoplastic polymeric MP pads using supercritical fluid according tothe invention; and

FIG. 6 is a cross-sectional diagram of an exemplary extrusion system forproducing foamed thermoplastic polymeric MP pads according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The invention uses known methods of fabricating foamed thermoplasticpolymeric materials using supercritical fluids to fabricatethermoplastic polymeric mechanical planarization (“MP”) polishing pads.

FIG. 1 shows an exemplary method 100 for fabricating foamedthermoplastic polymeric MP pads using supercritical fluid in accordancewith the invention. At step 102, a single-phase solution consistingessentially of a molten thermoplastic polymer and a supercritical fluidfoaming agent is formed. A supercritical fluid is a material that isconcurrently maintained at temperatures and pressures exceeding thecritical temperature (T_(c)) and critical pressure (P_(c)) of thematerial. The supercritical fluid foaming agent is preferablysupercritical carbon dioxide (T_(c)=31.1° C., P_(c)=1071.3 psi) orsupercritical nitrogen (T_(c)=−147.0° C., P_(c)=492.3 psi), but may beany other supercritical fluid that is gaseous under obtainable MP padprocessing conditions and that readily dissolves in a thermoplasticpolymer. The thermoplastic polymer may be, for example, anythermoplastic elastomer (“TPE”) (e.g., thermoplastic polyurethane(“TPU”)), thermoplastic butadiene styrene, thermoplastic polyvinylidenedifluorine (“PVDF”), high-impact polystyrene (“HIPS”), any othersuitable thermoplastic polymer, or any combination of suitablethermoplastic polymers (i.e., polymer blends or copolymers). Because thepercent composition by weight of a supercritical fluid foaming agent ina single-phase solution of a thermoplastic polymer and supercriticalfluid foaming agent is generally directly proportional to the celldensity in a foamed thermoplastic polymer that results from thesingle-phase solution, and because it may be desirable to predeterminethe cell density in a foamed thermoplastic polymeric MP pad, a stream ofsupercritical fluid foaming agent and a stream of molten thermoplasticpolymer are preferably admixed in a predetermined ratio (e.g., less thanabout 15% composition by weight of the supercritical fluid foaming agentin the single-phase solution of the molten thermoplastic polymer andsupercritical fluid foaming agent) in the formation of a single-phasesolution at step 102. Note that in some embodiments, materials such asplasticizers, fillers, and woven and non-woven fabrics may be added tothe single-phase solution of step 102 to provide further advantageouscharacteristics in produced MP pads (e.g., adding plasticisers to softenproduced pads, adding fillers to stiffen produced pads, using fabrics tofurther strengthen and/or impart porosity in produced pads, etc.).

FIG. 2 shows cell densities for an exemplary foamed thermoplasticpolymer versus percent composition by weight of an exemplarysupercritical fluid foaming agent in a single-phase solution of thecorresponding thermoplastic polymer and supercritical fluid foamingagent. In particular, FIG. 2 is a graph of cell densities for foamedhigh-impact polystyrene (“HIPS”) versus percent composition by weight ofa supercritical fluid carbon dioxide foaming agent in a single-phasesolution of high-impact polystyrene and that foaming agent. As shown,the cell density of the foamed HIPS is directly proportional to thepercent composition by weight of the carbon dioxide foaming agent inthat single-phase solution. Similar proportionalities are obtained withother suitable thermoplastic polymers and thermoplastic polymer-solublesupercritical fluid foaming agents (e.g., supercritical fluid nitrogen).

The single-phase solution formed at step 102 is characteristicallythermodynamically unstable. Particularly, the solubility (S) of thesupercritical fluid foaming agent in the thermoplastic polymer issignificantly dependent on process temperature (T_(p)) and processpressure (P_(p)) FIG. 3 shows the solubility (y axis) of a supercriticalfluid foaming agent in a thermoplastic polymer versus processtemperature (z axis) and process pressure (x axis). As shown, thesolubility of a supercritical fluid foaming agent in a thermoplasticpolymer is inversely proportional to process temperature (i.e.,S∝1/T_(p)) and directly proportional to process pressure (i.e.,S∝P_(p)). At initial process temperature and pressure 302, an increasein pressure to pressure 304 (while T_(p) is fixed) increases solubilityof the supercritical fluid foaming agent in the thermoplastic polymer.In contrast, an increase in temperature from initial process temperatureand pressure 302 to temperature 306 (while P_(p) is fixed) decreasessolubility of the supercritical fluid foaming agent in the thermoplasticpolymer.

Because a supercritical fluid foaming agent is generally more soluble ina thermoplastic polymer than is a gaseous or liquid foaming agent,process temperature is preferably maintained above the criticaltemperature of the foaming agent and process pressure is preferablymaintained above the critical pressure of the foaming agent (thecombination of which places the foaming agent in a supercritical state)during formation of a single-phase solution at step 102. Further,because the solubility of the supercritical fluid foaming agent isinversely proportional to process temperature, and because a moltenthermoplastic polymer more readily dissolves a foaming agent than does asolid thermoplastic polymer, process temperature is preferablymaintained slightly above the melting point of the thermoplastic polymerduring formation of the single-phase solution at step 102. This achievesmaximum solubility of the supercritical fluid foaming agent in thethermoplastic polymer.

Returning to FIG. 1, the single-phase solution of the thermoplasticpolymer and the supercritical fluid foaming agent is advantageouslyhomogeneously nucleated at step 104. Homogeneous nucleation is a processby which supercritical fluid foaming agent molecules dissolved in themolten thermoplastic polymer assemble into uniformly sized clusters(e.g., generally including at least several molecules of the foamingagent). These clusters are evenly dispersed throughout the moltenthermoplastic polymer and define “nucleation sites” from which cellsthat impart porosity in produced thermoplastic polymeric MP pads grow.Because the homogeneously nucleated thermoplastic polymeric solutioncontains uniformly sized and evenly dispersed clusters of supercriticalfluid foaming agent, and because cells that impart porosity in producedMP pads grow from these clusters, uniformly porous MP pads can beadvantageously produced from the homogeneously nucleated solution.

Homogeneous nucleation of the single-phase solution is driven by thethermodynamic instability of the supercritical fluid foaming agent. Inparticular, homogeneous nucleation of the single-phase solution of thethermoplastic polymer and supercritical fluid foaming agent is inducedby rapidly varying, either individually or concurrently, processtemperature and process pressure such that the solubility of thesupercritical fluid foaming agent in the thermoplastic polymer isdecreased (i.e., by increasing temperature, decreasing pressure, orboth). For example, a quickly administered temperature increase induceshomogeneous nucleation of the single-phase solution. Alternatively orconcurrently, a quickly administered pressure drop (e.g., a drop inpressure of no less than about 1000 psi delivered at a rate of no lessthan about 14,500 psi/sec) also induces homogeneous nucleation of thesingle-phase solution.

The nucleation site density of a nucleated thermoplasticpolymer/supercritical fluid foaming agent material is generally directlyproportional to the pressure drop rate and to the temperature increaserate induced in the corresponding single-phase solution of the material(i.e., nucleation site density ∝−dP/dt and ∝dT/dt). Becausethermoplastic polymeric cells grow from the clusters of supercriticalfluid foaming agent that define the nucleation sites, as previouslydescribed, cell density in a foamed thermoplastic polymer is directlyproportional to the nucleation site density of the correspondingnucleated thermoplastic polymer (i.e., cell density ∝nucleation sitedensity). Therefore, the cell density of a foamed thermoplastic polymeris directly proportional to the pressure drop rate and to thetemperature increase rate induced in a single-phase solution of thecorresponding thermoplastic polymer and supercritical fluid foamingagent (i.e., cell density ∝−dP/dt and ∝dT/dt).

For example, FIG. 4 shows cell densities for an exemplary foamedthermoplastic polymer versus pressure drop rates induced in asingle-phase solution of the corresponding thermoplastic polymer and anexemplary supercritical fluid foaming agent. In particular, FIG. 4 showscell densities for foamed high-impact polystyrene (“HIPS”) versuspressure drop rates induced in a single-phase solution of high-impactpolystyrene and supercritical fluid carbon dioxide foaming agent. Thesolution is 10% percent composition by weight of the supercritical fluidcarbon dioxide. As shown, the cell density of the foamed HIPS isdirectly proportional to the pressure drop rate induced in thesingle-phase solution of the HIPS and supercritical fluid carbon dioxidefoaming agent. Similar proportionalities are obtained with othersuitable thermoplastic polymers and polymer-soluble supercritical fluidfoaming agents (e.g., supercritical fluid nitrogen).

Although admixing solid particle “nucleating agents” (e.g., talc,calcium carbonate, titanium oxide, barium sulfate, zinc sulfide, etc.)to a single-phase solution of a thermoplastic polymer and supercriticalfluid foaming agent can promote formation of additional nucleation sitesin the single-phase solution during nucleation, admixing solid particlenucleating agents to the single-phase solution of step 102 is generallyundesirable because solid particle nucleating agents can result inundesirable heterogeneous nucleation of the single-phase solution.Heterogeneous nucleation is the process by which supercritical fluidfoaming agent molecules dissolved in the molten thermoplastic polymerassemble into non-uniformly sized clusters that are unevenly dispersedthroughout the molten thermoplastic polymer. In particular, becausesolid particle nucleating agents generally induce a nonuniformity in thenucleation sites in a thermoplastic polymer (i.e., nonuniformity in sizeand nonuniformity in dispersion) and in the cells grown from thenucleation sites in the thermoplastic polymer, and because CMP pads arepreferably uniformly porous, admixing solid particle nucleating agentsto the single-phase solution of the thermoplastic polymer andsupercritical fluid foaming agent of step 102 is not recommended.

At step 106, cell growth begins at the nucleation sites. A pressure dropor temperature increase (e.g., induced at step 104 or at both steps 104and 106) results in expansion of the supercritical fluid foaming agentat step 106. In particular, because the volume of the supercriticalfluid foaming agent is directly proportional to temperature (i.e., V∝T)and inversely proportional to pressure (i.e., V∝1/P), a rapid increasein process temperature, or a rapid decrease in process pressure, orboth, results in a rapid increase in the volume of the supercriticalfluid foaming agent in the thermoplastic polymer. Expanding foamingagent forms-microcellular pores at the nucleation sites in thethermoplastic polymer, thus foaming the thermoplastic polymer. Dependingon process conditions (e.g., whether the process pressure exceeds thecritical pressure of the supercritical fluid foaming agent and whetherthe process temperature exceeds the critical temperature of thesupercritical fluid foaming agent), the expanding foaming agent can besupercritical or gaseous at step 106.

Process conditions (e.g., process pressure, pressure drop rate, processtemperature, and temperature increase rate) determine whether a foamedthermoplastic polymeric MP pad of the invention is open-celled,closed-celled, or a combination of both. Open-celled is a condition inwhich microcellular pores of a foamed thermoplastic polymer are mutuallyinclusive (i.e., the pore membrane of a microcellular pore is not intactand infringes on neighboring pore membranes). Foaming agent diffuses outof each open-celled microcellular pore in an open-celled MP pad.Closed-celled is a condition in which microcellular pores of a foamedthermoplastic polymer are mutually exclusive (i.e., the pore membrane ofa microcellular pore is intact and does not infringe on neighboring poremembranes). Foaming agent is trapped in the membrane of eachclosed-celled microcellular pore in a closed-celled MP pad.

In particular, process conditions that significantly exploit thethermodynamic instability of the supercritical fluid (i.e., a largepressure decrease at a quick pressure drop rate, a large temperatureincrease at a quick temperature increase rate, or both) generally resultin the formation of a predominantly open-celled foamed thermoplasticpolymer. Less violent process conditions (i.e., a moderate pressure dropat a moderate pressure drop rate, a moderate temperature increase at amoderate temperature increase rate, or both) generally result in theformation of a predominantly closed-celled foamed thermoplastic polymer.Mid-range process conditions generally result in the formation of afoamed thermoplastic polymer that is partially open-celled and partiallyclosed-celled.

In some embodiments (e.g., CMP processes), MP pads are preferablypredominantly open-celled because open-celled MP pads can provide anincreased transfer of polishing liquid (e.g., a fluid-based chemical)and thus an increased rate of material removal. In some embodiments, MPpads are preferably predominantly closed-celled because closed-celled MPpads can limit the flow of polishing liquid (e.g., inert polishingliquid) to the interface of the MP pad and an integrated circuit waferand thus carry away mechanically ground integrated circuit material.

At step 108, a foamed thermoplastic polymeric MP pad is formed from thefoamed thermoplastic polymer. Preferably, a foamed thermoplasticpolymeric MP polishing pad is molded from the foamed thermoplasticpolymer. For example, an extruder can extrude a foamed thermoplasticpolymer into an MP pad mold at step 108. The foamed thermoplasticpolymer in the MP pad mold is then exposed to temperatures and pressuresto either promote or prevent continued cell growth in the foamedthermoplastic polymer. In particular, the foamed thermoplastic polymerin the MP pad mold can be exposed to low temperatures and high pressuresto prevent continued cell growth or exposed to high temperatures and lowpressures to promote continued cell growth in the foamed thermoplasticpolymer. Alternatively, the foamed thermoplastic polymer in the CMP padmold can be exposed to ambient conditions, which will cause cell growthin the foamed thermoplastic polymer to gradually stop.

In other embodiments, the foamed thermoplastic polymer is formed into asheet at step 108, and individual foamed thermoplastic polymeric MP padsare subsequently mechanically skived from the sheet. However, becausemechanical skiving can cause MP pad defects, as previously described, itmay be less desirable to form the foamed thermoplastic polymer into asheet from which individual foamed thermoplastic polymeric MP pads aremechanically skived. Nonetheless, because mechanically-skivedthermoplastic polymeric MP pads may be planarized or further finished toreduce pad defects caused by mechanical skiving, and because somemolding processes may introduce surface contamination (e.g., caused bymold-release compounds that facilitate the removal of MP pads from an MPmold), the surface of mechanically-skived, planarized MP pads may be, insome instances, more uniform than the surface of molded MP pads.

In another method of fabricating foamed thermoplastic polymeric MP padsin accordance with the invention, a foamed thermoplastic polymeric MPpad is formed in a single step (i.e., steps 104, 106, and 108 performedconcurrently) from a single-phase solution of the correspondingthermoplastic polymer and a supercritical fluid. A pressurizedsingle-phase solution of a thermoplastic polymer and a supercriticalfluid is extruded into an MP pad mold that is maintained at ambientpressure conditions. A rapid pressure drop nucleates the single-phasesolution and causes the foaming agent to expand, thus growing cells inthe thermoplastic polymer. In sum, the single-phase solution can behomogeneously nucleated, cells can grow in the thermoplastic polymer,and the MP pad can be formed in the same process step.

FIG. 5 shows another exemplary method 500 of fabricating foamedthermoplastic polymeric MP pads in accordance with the invention. Atstep 502, a solid thermoplastic polymer is formed. The thermoplasticpolymer may be any of the thermoplastic polymers, polymer blends, orcopolymers suitable for use in method 100. Additionally, fillers,plasticisers, and woven and non-woven fabrics may be added duringthermoplastic polymer formation at step 502 to provide furtheradvantageous characteristics in produced MP pads. In some embodiments, athermoplastic polymer is molded in an MP pad mold at step 502. In otherembodiments, thermoplastic polymer resins are melted to form a sheet atstep 502 from which individual thermoplastic polymer MP pads aresubsequently mechanically skived. Mechanical skiving of thethermoplastic polymeric sheet can be performed before or after the sheetof thermoplastic polymer is foamed. As previously described, methods offabricating foamed MP pads that require mechanical skiving are generallyless desirable because mechanical skiving can cause pad defects.However, mechanically-skived MP pads may be planarized or furtherfinished to reduce MP pad defects.

At step 504, the thermoplastic polymer is impregnated with supercriticalfluid foaming agent. The supercritical fluid foaming agent may be any ofthe supercritical fluid foaming agents suitable for use in method 100.Particularly, the supercritical fluid foaming agent is preferablysupercritical carbon dioxide or supercritical nitrogen, but can be anyother supercritical fluid that is gaseous under ambient conditions andthat readily dissolves in a thermoplastic polymer. Because asupercritical fluid foaming agent is generally more soluble in athermoplastic polymer than is a gaseous or liquid foaming agent, at step504, process temperature is preferably maintained above the criticaltemperature of the foaming agent and process pressure is preferablymaintained above the critical pressure of the foaming agent (whichplaces the foaming agent in a supercritical state). Also at step 504,the thermoplastic polymer is placed in a pressurized chamber and bathedin a supercritical fluid foaming agent. This causes the thermoplasticpolymer to become saturated with the supercritical fluid foaming agent.

At step 506, the thermoplastic polymer is foamed. In particular, thesolubility and volume of the supercritical fluid foaming agent in thethermoplastic polymer are rapidly changed to cause nucleation and cellgrowth in the thermoplastic polymer. Expanding clusters of foaming agentmolecules form microcellular pores in the thermoplastic polymer, thusfoaming the thermoplastic polymer. In some embodiments, a single, quickpressure drop (e.g., a drop in pressure of no less than about 1000 psidelivered at a rate of no less than about 14,500 psi/sec), or a single,quick increase in temperature, or both, causes a rapid decrease in thesolubility and a rapid increase in the volume of the supercritical fluidfoaming agent in the thermoplastic polymer. In other embodiments, afirst moderate pressure drop or temperature increase (or both) causes arapid decrease in the solubility of the supercritical fluid foamingagent in the thermoplastic polymer and a second moderate pressure dropor temperature increase (or both) causes a rapid increase in the volumeof the supercritical fluid foaming agent in the thermoplastic polymer.As previously described, process conditions (e.g., the level and rate ofpressure drop and the level and rate of temperature increase) generallydetermine whether foamed thermoplastic polymeric MP pads areopen-celled, closed-celled, or a combination of both.

FIG. 6 shows an exemplary extrusion system 600 for producing foamedthermoplastic polymeric MP pads in accordance with the invention. Inextrusion system 600, extrusion screw 602 is positioned inside a firstregion of extruder casing 604. Extrusion screw 602 has a plurality ofscrew flights 606 that facilitate downstream movement of thermoplasticpolymer and foaming agent injected into extruder casing 604 bythermoplastic polymer source 608 and foaming agent source 610.Downstream movement is in the direction indicated by arrows 612 and 614.As shown, some screw flights 606 are discontinuous (i.e., contain openspaces). Discontinuous screw flights 616 and 618 facilitate mixing ofthe thermoplastic polymer and foaming agent. In particular,discontinuous screw flights 616 and 618 facilitate formation of asingle-phase solution of the thermoplastic polymer and supercriticalfluid foaming agent. Drive motor 620 rotates extrusion screw 602 duringthe extrusion process.

Temperature control units 622 are positioned along the exterior ofextruder casing 604 and control the extrusion process temperature.Because the solubility of a supercritical fluid foaming agent isinversely proportional to the process temperature, and because a moltenthermoplastic polymer more readily dissolves the foaming agent than doesa solid thermoplastic polymer, temperature control units 622 preferablymaintain process temperature slightly above the melting point of thethermoplastic polymer during formation of a single-phase solution of athermoplastic polymer and supercritical fluid. Temperature control units622 may extend along the entire exterior of extruder casing 604 or maybe selectively positioned along the exterior of extruder casing 604.Individual temperature control units 622 may maintain a common processtemperature in all regions of extruder casing 604, or may maintaindifferent process temperatures in different regions of extruder casing604. For example, because a rapid increase in temperature can result innucleation of a single-phase solution of a thermoplastic polymer andsupercritical fluid foaming agent, temperature control units 622 maymaintain a differential temperature between adjacent regions of externalcasing 604 to induce nucleation as the single-phase solution passes fromone region of external casing 604 to an adjacent region.

Pressure control units 624 are positioned along the exterior of extrudercasing 604 and control the extrusion process pressure. Because thesolubility of a supercritical fluid foaming agent is directlyproportional to the process pressure, and because a supercritical fluidfoaming agent is more soluble in a thermoplastic polymer than is aliquid or gaseous foaming agent, pressure control units 624 preferablymaintain process pressure above the critical pressure of thesupercritical fluid foaming agent during formation of a single-phasesolution of a thermoplastic polymer and supercritical fluid foamingagent. Pressure control units 624 may extend along the entire exteriorof extruder casing 604 or may be selectively positioned along theexterior of extruder casing 604. Individual pressure control units 624may maintain a common process pressure in all regions of extruder casing604, or may maintain different process pressures in different regions ofextruder casing 604. For example, because a rapid drop in pressure canresult in nucleation of a single-phase solution of a thermoplasticpolymer and supercritical fluid foaming agent, pressure control units624 may maintain a differential pressure between adjacent regions ofexternal casing 604 to induce nucleation as the single-phase solutionpasses from one region of external casing 604 to an adjacent region.

Nucleator 626 is positioned in a second region of extruder casing 604.Nucleator 626 is a collection of restrictive pathways through which thesingle-phase solution formed in the first region of casing 604experiences a pressure drop that induces homogeneous nucleation of thesingle-phase solution. Particularly, as the single-phase solution of athermoplastic polymer and supercritical fluid foaming agent passesthrough the restrictive pathways of nucleator 626, a rapid pressure dropcauses a decrease in solubility of the supercritical fluid foaming agentin the thermoplastic polymer. Uniformly sized and evenly dispersedclusters of foaming agent molecules form in the thermoplastic polymer,thus homogeneously nucleating the thermoplastic polymer. In someembodiments, nucleator 626 is a temperature control unit that induces arapid temperature increase in the single-phase solution formed in thefirst region of extruder casing 604. In such extrusion systems, therapid temperature increase induces nucleation of the single-phasesolution of a thermoplastic polymer and supercritical fluid foamingagent.

A third region of extruder casing 604 forms chamber 628, which isimmediately downstream of nucleator 626. Chamber 628 receives nucleatedthermoplastic polymer from nucleator 626. The temperature control units622 and the pressure control unit 624 positioned along the exterior ofthe third region of extruder casing 604 control the temperature andpressure, respectively, in chamber 628. In particular, temperaturecontrol units 622 and pressure control unit 624 positioned along theexterior of chamber 628 control cell growth in the nucleatedthermoplastic polymer passed from nucleator 626 into chamber 628.Because a pressure drop can induce cell growth in a nucleatedthermoplastic polymer, the pressure control unit 624 that controlsprocess pressure in chamber 628 may optionally maintain pressure inchamber 628 significantly lower than the process pressure upstream ofnucleator 626 (e.g., no less than 1000 psi lower). Because a temperatureincrease can induce cell growth in the nucleated thermoplastic polymer,the temperature control units 622 that control process temperature inchamber 628 may optionally maintain temperature in chamber 628significantly higher than the process temperature upstream of nucleator626 to promote cell growth in the thermoplastic polymer.

Downstream from chamber 628, foamed thermoplastic polymer is extrudedfrom extrusion system 600 to MP pad formation means 630. In someembodiments, MP pad formation means is an MP pad mold. The foamedthermoplastic polymer in the MP pad mold can be exposed to temperaturesand pressures that control continued cell growth in the foamedthermoplastic polymer. Alternatively, the foamed thermoplastic polymerin the MP pad mold can be exposed to ambient conditions, which willcause cell growth in the foamed thermoplastic polymer to gradually stop.

In other embodiments, MP pad formation means 630 is a machine (e.g., ablow molding machine) for forming the foamed thermoplastic polymer intoa sheet from which individual MP polishing pads are mechanically skived.However, because mechanical skiving can cause MP pad defects, aspreviously described, it is generally less desirable to form a sheet offoamed thermoplastic material from which individual MP pads aremechanically skived.

In alternative embodiments in accordance with the invention, chamber 628can be excluded from extrusion system 600. Foamed thermoplastic materialis instead extruded directly from nucleator 626 to MP pad formationmeans 630. For example, the foamed thermoplastic material can beextruded directly from nucleator 626 into an MP pad mold. The MP padmold can be maintained at ambient pressure and temperature, oralternatively at pressures and temperatures that control cell growth inthe thermoplastic polymer. Also, foamed thermoplastic polymer may beextruded directly from nucleator 626 and formed into a sheet (e.g.,extruding directly into sheet form or using a blow molding machine) fromwhich individual MP pads are mechanically cut.

Thus it is seen that known methods of fabricating foamed thermoplasticpolymers using supercritical fluids can be used to fabricate foamedthermoplastic polymeric MP pads. One skilled in the art will appreciatethat the invention can be practiced by other than the describedembodiments, which are presented for purposes of illustration and not oflimitation, and the invention is limited only by the claims whichfollow.

I claim:
 1. Apparatus for fabricating foamed thermoplastic polymericmechanical planarization polishing pads using supercritical fluid, theapparatus comprising: a mechanical planarization polishing pad mold; anextrusion system used for molding a solid thermoplastic polymer in amechanical planarization polishing pad mold to form an unfoamedthermoplastic polymeric pad; an extrusion screw used for impregnatingsaid unfoamed thermoplastic polymeric pad with a supercritical fluidfoaming agent; a plurality of discontinuous screw flights of saidextrusion screw for mixing said unfoamed thermoplastic polymeric padwith said supercritical fluid foaming agent to achieve a desired celldensity of a foamed thermoplastic polymeric mechanical planarizationpolishing pad; a foaming chamber; a nucleator comprising a plurality ofrestrictive pathways positioned between the extrusion screw and thefoaming chamber; and a temperature control unit and a pressure controlunit, positioned on the exterior of said foaming chamber, used forfoaming said unfoamed thermoplastic polymeric pad to produce said foamedthermoplastic polymeric mechanical planarization polishing pad, whereinin response to said desired cell density, said temperature control unitand said pressure control unit are set to: a process temperature; aprocess pressure; a rate of temperature increase; and a pressure droprate.
 2. Apparatus for fabricating foamed thermoplastic polymericmechanical planarization polishing pads using supercritical fluid, theapparatus comprising: a mechanical planarization polishing pad mold; anextrusion system used for molding a solid thermoplastic polymer in amechanical planarization polishing pad mold to form an unfoamedthermoplastic polymeric pad; an extrusion screw used for impregnatingsaid unfoamed thermoplastic polymeric pad with a supercritical fluidfoaming agent; and a plurality of discontinuous screw flights of saidextrusion screw for mixing said unfoamed thermoplastic polymeric padwith said supercritical fluid foaming agent to achieve a desired celldensity of a foamed thermoplastic polymeric mechanical planarizationpolishing pad; a foaming chamber; a nucleator comprising a plurality ofrestrictive pathways positioned between the extrusion screw and thefoaming chamber; and a temperature control unit and a pressure controlunit, positioned on the exterior of said foaming chamber, used forfoaming said unfoamed thermoplastic polymeric pad to produce a foamedthermoplastic polymeric mechanical planarization polishing pad bydecreasing process pressure and increasing process temperature of saidsupercritical fluid foaming agent, wherein in response to said desiredcell density, said temperature control unit and said pressure controlunit are set to: a process temperature; a process pressure; a rate oftemperature increase; and a pressure drop rate.
 3. The apparatus ofclaim 1 wherein said solid thermoplastic polymer is selected from thegroup consisting of thermoplastic elastomer, thermoplastic butadienestyrene, thermoplastic polyvinylidene difluorine, high-impactpolystyrene, and any combination thereof.
 4. The apparatus of claim 1wherein said supercritical fluid foaming agent is selected from thegroup consisting of supercritical fluid carbon dioxide and supercriticalfluid nitrogen.
 5. The apparatus of claim 1 wherein said extrusion screwused for impregnating said unfoamed thermoplastic polymeric pad isconfigured to impregnate said unfoamed thermoplastic polymeric pad witha supercritical fluid foaming agent at a process pressure that exceeds acritical pressure of said supercritical fluid foaming agent.
 6. Theapparatus of claim 1 wherein said extrusion screw used for impregnatingsaid unfoamed thermoplastic polymeric pad is configured to impregnatesaid unfoamed thermoplastic polymeric pad with a supercritical fluidfoaming agent at a process temperature that exceeds a criticaltemperature of said supercritical fluid foaming agent.
 7. The apparatusof claim 1 wherein said extrusion screw used for impregnating saidunfoamed thermoplastic polymeric pad is configured to: place saidunfoamed thermoplastic polymeric pad in a pressurized chamber whereinprocess pressure in said pressurized chamber exceeds a critical pressureof said supercritical fluid foaming agent and process temperature insaid pressurized chamber exceeds a critical temperature of saidsupercritical fluid foaming agent; and bathe said unfoamed thermoplasticpolymeric pad in said supercritical fluid foaming agent while saidunfoamed thermoplastic polymeric pad is in said pressurized chamber. 8.The apparatus of claim 1 wherein said temperature control unit and saidpressure control unit used for foaming said unfoamed thermoplasticpolymeric pad are configured to: decrease the solubility of saidsupercritical fluid foaming agent in said unfoamed thermoplasticpolymeric pad; and increase the volume of said supercritical fluidfoaming agent in said unfoamed thermoplastic polymeric pad.
 9. Theapparatus of claim 8 wherein said pressure control unit is furtherconfigured to decrease process pressure of said supercritical fluidfoaming agent in said unfoamed thermoplastic polymeric pad.
 10. Theapparatus of claim 8 wherein said temperature control unit is furtherconfigured to increase process temperature of said supercritical fluidfoaming agent in said unfoamed thermoplastic polymeric pad.
 11. Theapparatus of claim 8 wherein said temperature control unit and saidpressure control unit are further configured to: decrease processpressure of said supercritical fluid foaming agent in said solidthermoplastic polymer; and increase process temperature of saidsupercritical fluid foaming agent in said unfoamed thermoplasticpolymeric pad.
 12. The apparatus of claim 8 wherein said pressurecontrol unit is further configured to decrease process pressure of saidsupercritical fluid foaming agent in said unfoamed thermoplasticpolymeric pad.
 13. The apparatus of claim 8 wherein said temperaturecontrol unit is further configured to increase process temperature ofsaid supercritical fluid foaming agent in said unfoamed thermoplasticpolymeric pad.
 14. The apparatus of claim 8 wherein said temperaturecontrol unit and said pressure control unit are further configured to:decrease process pressure of said supercritical fluid foaming agent insaid unfoamed thermoplastic polymeric pad; and increase processtemperature of said supercritical fluid foaming agent in said unfoamedthermoplastic polymeric pad.
 15. The apparatus of claim 1 wherein saidpressure control unit is configured to decrease process pressure of saidsupercritical fluid foaming agent in said unfoamed thermoplasticpolymeric pad.
 16. The apparatus of claim 1 wherein said temperaturecontrol unit is configured to increase process temperature of saidsupercritical fluid foaming agent in said unfoamed thermoplasticpolymeric pad.
 17. The apparatus of claim 1 wherein said temperaturecontrol unit and said pressure control unit are configured to: decreaseprocess pressure of said supercritical fluid foaming agent in saidunfoamed thermoplastic polymeric pad; and increase process temperatureof said supercritical fluid foaming agent in said unfoamed thermoplasticpolymeric pad.
 18. The apparatus of claim 1 wherein said temperaturecontrol unit and said pressure control unit used for foaming saidunfoamed thermoplastic polymeric pad are configured to foam saidunfoamed thermoplastic polymeric pad to produce an open-celled foamedthermoplastic polymeric mechanical planarization polishing pad.
 19. Theapparatus of claim 1 wherein said temperature control unit and saidpressure control unit used for foaming said unfoamed thermoplasticpolymeric pad are configured to foam said unfoamed thermoplasticpolymeric pad to produce an closed-celled foamed thermoplastic polymericmechanical planarization polishing pad.
 20. An apparatus for fabricatingfoamed thermoplastic polymeric mechanical planarization polishing pads,comprising: an extruder casing; an extrusion screw positioned within afirst region of the extruder casing, the extrusion screw comprising aplurality of unidirectional screw flights at least some of which arediscontinuous screw flights; one or more temperature control unitspositioned along the exterior of the extruder casing; one or morepressure control units positioned along the exterior of the extrudercasing; a nucleator comprising a plurality of restrictive pathwayspositioned within a second region of the extruder casing; and amechanical planarization polishing pad formation device selected fromthe group consisting of a mechanical planarization polishing pad moldand a blow molding machine.
 21. The apparatus of claim 20 wherein themechanical planarization polishing pad formation device is immediatelydownstream from the nucleator.
 22. The apparatus of claim 20 furthercomprising a foaming chamber within a third region of the casingpositioned between the nucleator and the mechanical planarizationpolishing pad formation device.
 23. The apparatus of claim 20 wherein asingle-phase solution formed in the first region of the casing undergoesa pressure drop that induces homogenous nucleation as it passes thoughthe nucleator.
 24. The apparatus of claim 20 wherein a single-phasesolution formed in the first region of the casing undergoes atemperature increase that induces homogenous nucleation as it passesthough the nucleator.
 25. The apparatus of claim 20 wherein the one ormore temperature control units are configured to maintain a temperaturedifferential between adjacent regions of the extruder casing.
 26. Theapparatus of claim 20 wherein the one or more temperature control unitsare configured to maintain a common process temperature in all regionsof the extruder casing.
 27. The apparatus of claim 20 wherein the one ormore temperature control units extend along the entire exterior of theextruder casing.
 28. The apparatus of claim 20 wherein the one or morepressure control units extend along the entire exterior of the extrudercasing.
 29. The apparatus of claim 20 wherein the one or more pressurecontrol units are configured to maintain different process pressures indifferent regions of the extruder casing.